EP2637239B1 - Porous electrode base material and process for production thereof, porous electrode base material precursor sheet, membrane-electrode assembly, and solid polymer fuel cell - Google Patents

Porous electrode base material and process for production thereof, porous electrode base material precursor sheet, membrane-electrode assembly, and solid polymer fuel cell Download PDF

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Publication number
EP2637239B1
EP2637239B1 EP11837908.0A EP11837908A EP2637239B1 EP 2637239 B1 EP2637239 B1 EP 2637239B1 EP 11837908 A EP11837908 A EP 11837908A EP 2637239 B1 EP2637239 B1 EP 2637239B1
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EP
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Prior art keywords
carbon fibers
short carbon
porous electrode
electrode substrate
dimensional
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EP11837908.0A
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German (de)
English (en)
French (fr)
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EP2637239A1 (en
EP2637239A4 (en
Inventor
Kazuhiro Sumioka
Yoshihiro Sako
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Mitsubishi Rayon Co Ltd
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Mitsubishi Rayon Co Ltd
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    • H—ELECTRICITY
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    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8647—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites
    • H01M4/8657—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites layered
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B18/00—Layered products essentially comprising ceramics, e.g. refractory products
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    • C04B35/71—Ceramic products containing macroscopic reinforcing agents
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    • C04B35/83—Carbon fibres in a carbon matrix
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    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
    • D21H13/00—Pulp or paper, comprising synthetic cellulose or non-cellulose fibres or web-forming material
    • D21H13/36—Inorganic fibres or flakes
    • D21H13/46—Non-siliceous fibres, e.g. from metal oxides
    • D21H13/50—Carbon fibres
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
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    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
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Definitions

  • the present invention relates to a process of producing a porous electrode substrate used in polymer electrolyte fuel cells.
  • Polymer electrolyte fuel cells are characterized in using a proton conductive polymer electrolyte membrane, and are devices that obtain electromotive force by allowing oxidizing gas such as oxygen to electrochemically react with fuel gas such as hydrogen.
  • Such polymer electrolyte fuel cells have a structure in which two gas diffusion electrodes, including a catalyst layer with carbon powder on which a noble metal-based catalyst is supported as a main component and a gas diffusion electrode base material, respectively have a catalyst layer side thereof set to the inside, and are bonded to both sides of a polymer electrolyte membrane.
  • the gas diffusion electrode substrate is generally configured from a carbonaceous material, and the gas diffusion electrode substrates indicated below are known, for example.
  • Patent Document 1 discloses a porous carbon electrode substrate for fuel cells characterized by having a thickness of 0.05 to 0.5 mm, a bulk density of 0.3 to 0.8 g/cm 3 , and a bending strength of at least 10 MPa and deflection upon bending of at least 1.5 mm in a three-point bending test under the conditions of a strain rate of 10 mm/min, a distance between support points of 2 cm and a test piece width of 1 cm.
  • Patent Document 2 discloses a gas diffusion layer for fuel cells including a mat having a plurality of carbon fibers, and a plurality of acrylic pulp fibers incorporated into this carbon fiber mat, and that the acrylic pulp fibers are cured and carbonized after incorporation into the carbon fiber mat.
  • Patent Document 3 discloses a carbon fiber sheet having a thickness of 0.15 to 1.0 mm, a bulk density of 0.15 to 0.45 g/cm 3 , a carbon fiber content of at least 95% by mass, a compressive deformation rate of 10 to 35%, an electrical resistance value of no more than 6 m ⁇ , and a degree of drape of 5 to 70 g.
  • Patent Document 4 discloses a carbon fiber non-woven fabric for a polyelectrolyte fuel cell electrode material having a thickness of 0.15 to 0.60 mm, a basis weight of 50 to 150 g/m 2 , a specific resistance value in the thickness direction of no more than 0.20 ⁇ cm, and a surface pile number of no more than 15/mm 2 .
  • Patent Document 1 Pamphlet of PCT International Publication No. WO2001/056103
  • Patent Document 2 Japanese Unexamined Patent Application, Publication No. 2007-273466
  • Patent Document 3 Pamphlet of PCT International Publication No. WO2002/042534
  • Patent Document 4 Japanese Unexamined Patent Application, Publication No. 2003-45443
  • US 20050150620 discloses a carbon fiber paper including carbon fibers having a surface area ratio of 1.05 or more, and a porous carbon electrode substrate for a fuel cell having this carbon fiber paper as a constituent.
  • the electrode substrate for a fuel cell has uniformly dispersed carbon fibers and is flexible. Furthermore, it is disclosed that the carbon fiber paper is suitable for production of the electrode substrate.
  • EP 2506353 discloses a porous electrode substrate that has large sheet strength, low production costs, high handling properties, high thickness precision and surface smoothness, and sufficient gas permeability and electrical conductivity.
  • This disclosure also relates to a porous electrode substrate including a three-dimensional entangled structure including short carbon fibers (A) dispersed in a three-dimensional structure, joined together via three-dimensional mesh-like carbon fibers (B).
  • a method for producing a porous electrode substrate including a step (1) of producing a precursor sheet including short carbon fibers (A), and short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') dispersed in a two-dimensional plane; a step (2) of subjecting the precursor sheet to entanglement treatment; and a step (3) of subjecting this sheet to carbonization treatment at 1000°C or higher is also disclosed.
  • the porous carbon electrode material disclosed in Patent Document 1 has high mechanical strength and surface smoothness, and has adequate gas permeability and electrical conductivity, the production cost thereof is high.
  • the gas diffusion layer for fuel cells disclosed in Patent Document 2 can be produced at low cost, there is little entangling of the acrylic pulp with carbon fibers upon making into a sheet, and thus the handling thereof has been difficult.
  • acrylic pulp since acrylic pulp has low molecular orientation of polymer compared with fibrous materials, the carbonization rate during carbonization is low, and thus it has been necessary to add an abundance of acrylic pulp in order to raise the handling property.
  • the carbon fiber sheet and carbon fiber non-woven fabric disclosed in Patent Documents 3 and 4 can be produced at low cost; however, the shrinkage thereof during calcining is large, and thus the undulation (heaving state or warped state of sheet cross section) of the obtained sheet, etc. has been large. Furthermore, since the handling property is improved by fiber entangling, the sheet surface has become fluffy from the carbon fibers, whereby the polymer electrolyte membrane has been damaged upon incorporating the sheet in the fuel cell.
  • the present invention has an object of providing a porous electrode substrate excelling in handling property, having improved sheet undulation, as well as retaining sufficient gas permeability and electrical conductivity, and further, does not inflict damage on the polymer electrolyte membrane upon incorporating in the fuel cell, as well as a process of producing the same.
  • the present invention has an object of providing a porous electrode substrate precursor sheet that can be suitably used in order to obtain the above-mentioned porous electrode substrate, as well as a membrane-electrode assembly and polymer electrolyte fuel cell using the above-mentioned porous electrode substrate.
  • a porous electrode substrate includes, layer stacked and integrated therein: a three-dimensional structure Y-1 produced by bonding short carbon fibers (A1) by carbon (D); and a three-dimensional structure Y-2 produced by bonding short carbon fibers (A2) by carbon (D), in which the short carbon fibers (A1) form a three-dimensional entangled structure in the three-dimensional structure Y-1, and the short carbon fibers (A2) do not form a three-dimensional entangled structure in the three-dimensional structure Y-2, wherein a three-dimensional entangled structure is a structure in which the average of the angles of the short carbon fibers with the horizontal plane of the sheet surface is measured as being at least 3°, or the maximum value of the angle between the short carbon fibers and the horizontal plane is measured as being at least 10°, and wherein carbon D is carbon having a mesh-like form, or a carbonized resin, or a combination of these.
  • the three-dimensional structure Y-1 is a three-dimensional structure in which the short carbon fibers (A1) are bonded by three-dimensional mesh-like carbon fibers (B), and the three-dimensional structure Y-2 is a three-dimensional structure in which the short carbon fibers (A2) are bonded by two-dimensional mesh-like carbon fibers (C), wherein a mesh-like structure has a bent shape or a curved shape at the bonding part, wherein three-dimensional mesh-like carbon fibers have an average angle of the mesh-like constituent carbon fibers with a horizontal plane of at least 3°, wherein two-dimensional mesh-like carbon fibers have an average angle of the mesh-like constituent carbon fibers with a horizontal plane of less than 2°.
  • a porous electrode substrate precursor sheet includes, layer stacked and integrated therein: a precursor sheet X-2' having dispersed therein short carbon fibers (A1), and short carbon fiber precursors (b1) and/or fibrillar carbon fiber precursors (b1'); and a precursor sheet X-3' having dispersed therein short carbon fibers (A2), and short carbon fiber precursors (b2) and/or fibrillar carbon fiber precursors (b2'), in which the short carbon fibers (A1) form a three-dimensional entangled structure in the precursor sheet X-2', and the short carbon fibers (A2) do not form a three-dimensional entangled structure in the precursor sheet X-3'.
  • a mass ratio of a mass of the short carbon fibers (A1) to a total mass of the short carbon fiber precursors (b1) and the fibrillar carbon fiber precursors (b1') is 20:80 to 80:20
  • a mass ratio of a mass of the short carbon fibers (A2) to a total mass of the short carbon fiber precursors (b2) and the fibrillar carbon fiber precursors (b2') is 20:80 to 80:20.
  • a process of producing a porous electrode substrate includes: a step (1) of dispersing short carbon fibers (A1), and producing a precursor sheet X-1 of the short carbon fibers (A1) not having a three-dimensional entangled structure; a step (2) of obtaining a precursor sheet X-2 having a three-dimensional entangled structure of the short carbon fibers (A1), by entanglement treating the precursor sheet X-1; a step (3) of obtaining a porous electrode substrate precursor sheet X-4 by dispersing short carbon fibers (A2), and layer stacking and integrating a precursor sheet X-3 not having a three-dimensional entangled structure of the short carbon fibers (A-2), on the precursor sheet X-2; and a step (4) of carbonization treating the porous electrode substrate precursor sheet X-4 at a temperature of at least 1000°C, wherein entanglement treating is any method in which a three-dimensional entangled structure is formed.
  • the precursor sheet X-2 having a three-dimensional entangled structure of the short carbon fibers (A1) and the precursor sheet X-3 not having a three-dimensional entangled structure of the short carbon fibers (A2) are layer stacked and integrated in step (3), by feeding a slurry in which the short carbon fibers (A2) are dispersed in a liquid medium onto the precursor sheet X-2 and making into a sheet.
  • the short carbon fiber precursors (b1) and/or fibrillar carbon fiber precursors (b1') are dispersed together with the short carbon fibers (A1) in step (1), and the short carbon fiber precursors (b2) and/or fibrillar carbon fiber precursors (b2') are dispersed together with the short carbon fibers (A2) in step (3).
  • the process of producing a porous electrode substrate as described in any one of the fifth to seventh aspects further includes a step (5) of hot press molding the porous electrode substrate precursor sheet X-4 at a temperature of less than 200°C, after step (3) but before step (4) .
  • the process of producing a porous electrode substrate as described in the eighth aspect further includes a step (6) of oxidation treating, at a temperature of at least 200°C and less than 300°C, the porous electrode substrate precursor sheet X-4 subjected to hot press molding, after step (5) but before step (4).
  • a porous electrode substrate is obtained by the process as described in any one of the fifth to ninth aspects.
  • a membrane-electrode assembly includes the porous electrode substrate as described in the first, second or tenth aspect.
  • a polymer electrolyte fuel cell includes the membrane-electrode assembly as described in the eleventh aspect.
  • the present invention provides a porous electrode substrate excelling in handling property, having improved sheet undulation, as well as retaining sufficient gas permeability and electrical conductivity, and further, does not inflict damage on the polymer electrolyte membrane upon inserting in the fuel cell, as well as a process of producing the same.
  • the present invention provides a porous electrode substrate precursor sheet that can be suitably used in order to obtain the above-mentioned porous electrode substrate, as well as a membrane-electrode assembly and polymer electrolyte fuel cell using the above-mentioned porous electrode substrate.
  • a porous electrode substrate of the present invention is composed of a structure layer stacking and integrating a three-dimensional structure Y-1 made by bonding short carbon fibers (A1) by carbon (D) and a three-dimensional structure Y-2 made by bonding short carbon fibers (A2) by carbon (D).
  • the short carbon fibers (A1) form a three-dimensional entangled structure in the three-dimensional structure Y-1.
  • the short carbon fibers (A2) do not form a three-dimensional entangled structure in the three-dimensional structure Y-2.
  • the short carbon fibers (A1) and short carbon fibers (A2) (hereinafter may be collectively called "short carbon fibers (A)" may be the same or may be different.
  • the three-dimensional structure Y-1 is a three-dimensional structure made by bonding the short carbon fibers (A1) by carbon (D), and is a structure in which the short carbon fibers (A1) constituting the structure Y-1 are three-dimensionally entangled in the structure Y-1.
  • the three-dimensional structure Y-2 is a three-dimensional structure made by bonding the short carbon fibers (A2) by carbon (D), and is a structure in which the short carbon fibers (A2) constituting the structure Y-2 are not three-dimensionally entangled in the structure Y-2.
  • the three-dimensional structure Y-1 may be a three-dimensional structure in which the short carbon fibers (A1) are bonded by the three-dimensional mesh-like carbon fibers (B), and the three-dimensional structure Y-2 may be a three-dimensional structure in which the short carbon fibers (A2) are bonded by two-dimensional mesh-like carbon fibers (C).
  • the porous electrode substrate in which the three-dimensional structure Y-1 and the three-dimensional structure Y-2 are layer stacked and integrated can be in forms such as a sheet form and a spiral form.
  • the basis weight of the porous electrode substrate is preferably on the order of 15 to 100 g/m 2
  • the porosity is preferably on the order of 50 to 90%
  • the thickness is preferably on the order of 50 to 300 ⁇ m
  • the undulation is preferably no more than 5 mm.
  • the gas permeability of the porous electrode substrate is preferably 50 to 3000 ml/hr/cm 2 /Pa.
  • the electrical resistance (through-plane electrical resistance) in the thickness direction of the porous electrode substrate is preferably no higher that 50 m ⁇ cm 2 . It should be noted that the method of measuring the gas permeability and through-plane electrical resistance of the porous electrode substrate will be described later.
  • the total content of the three-dimensional mesh-like carbon fiber (B) to two-dimensional mesh-like carbon fiber (C) in the porous electrode substrate is preferably 5 to 90% by mass, and more preferably 10 to 60% by mass, from the viewpoint of the mechanical strength of the porous electrode substrate.
  • the content of short carbon fiber (A) in the porous electrode substrate is preferably 10 to 95% by mass, and more preferably 40 to 90% by mass.
  • whether or not the short carbon fibers (A) form a three-dimensional entangled structure can be determined by performing cross-sectional observation of the sheet-like measurement target (three-dimensional structure Y-1, three-dimensional structure Y-2, porous electrode substrate, precursor sheet X-2', precursor sheet X-3', porous electrode substrate precursor sheet, precursor sheet X-1, precursor sheet X-2, precursor sheet X-3), and measuring the angles between the respective short carbon fibers and the sheet surface in the cross section.
  • the cross section in which cross-sectional observation is made is a cross section in a vertical direction relative to the sheet surface of a sheet-like measurement target.
  • the short carbon fibers are forming a three-dimensional entangled structure (the measurement target has a three-dimensional entangled structure), and in a case of not being as such, it is determined that the short carbon fibers are not forming a three-dimensional entangled structure (the measurement target does not have a three-dimensional entangled structure). More specifically, using an SEM image of a cross section in the vertical direction relative to the sheet surface and drawing lines as indicated by the dotted lines on the short carbon fibers to be measured as in FIGS.
  • the lines 1 in FIGS. 5 and 6 are lines parallel to the sheet surface.
  • the number of measurement points upon deciding the average value and maximum value of angles can be set to 50 points, for example.
  • FIG. 7 A schematic diagram of a three-dimensional structure in which the short carbon fibers (A1) are bonded by the three-dimensional mesh-like carbon fibers (B) is shown in FIG. 7 .
  • the short carbon fibers (A1) are bonded by each of the carbon fibers 2 constituting the three-dimensional mesh-like carbon fibers (B).
  • FIG. 8 A schematic diagram of a three-dimensional structure in which the short carbon fibers (A2) are bonded by the two-dimensional mesh-like carbon fibers (C) is shown in FIG. 8 .
  • the short carbon fibers (A2) are bonded by the respective carbon fibers 3 constituting the two-dimensional mesh-like carbon fibers (C).
  • the determination of the mesh-like carbon fibers bonding the short carbon fibers being two-dimensional or three-dimensional can be conducted by carrying out cross section observation of a sheet-like measurement target (three-dimensional structure Y-1, three-dimensional structure Y-2), and measuring the angle in the cross section between the respective carbon fibers constituting the mesh-like carbon fibers bonding the short carbon fibers (carbon fibers 2 illustrated in FIG. 7 , carbon fibers 3 illustrated in FIG. 8 ), and the sheet surface.
  • the cross section in which cross section observation is performed is a cross section in a vertical direction relative to the sheet surface of the sheet-like measurement target.
  • “carbon fibers constituting the mesh-like carbon fibers bonding short carbon fibers" will be referred to as "mesh-like constituent carbon fibers”.
  • a case in which the average of the angle of the mesh-like constituent carbon fibers with a horizontal plane measured being at least 3° is determined as three-dimensional
  • a case of the average of the angle of the mesh-like constituent carbon fibers with a horizontal plane measured being less than 2° is determined as two-dimensional. More specifically, using an SEM image of a cross section in the vertical direction relative to the sheet surface and drawing dotted lines similarly to the dotted lines in FIGS. 5 and 6 on the mesh-like constituent carbon fibers being measured, it is sufficient to measure the angles between these lines and the sheet surface, similar to the measurement for the presence of a three-dimensional entangled structure. It should be noted that the number of measurement points upon deciding the average value of the angle can be set to 50 points, for example.
  • the short carbon fiber (A) one produced by cutting a carbon fiber such as polyacrylonitrile-based carbon fiber (hereinafter referred to as "PAN-based carbon fiber"), pitch-based carbon fiber and rayon-based carbon fiber to an appropriate length can be exemplified. From the viewpoint of the mechanical strength of the porous electrode substrate, PAN carbon fiber is preferable.
  • the average fiber length of the short carbon fibers (A) is preferably on the order of 2 to 12 mm in terms of dispersivity.
  • the average fiber diameter of the short carbon fibers (A) is preferably 3 to 9 ⁇ m in terms of the dispersivity of the short carbon fibers, and is more preferably 4 to 8 ⁇ m in terms of the smoothness of the porous electrode substrate.
  • the carbon (D) is used in order to bind between the short carbon fibers (A), and a carbide can be used as the carbon (D).
  • a carbide a carbonaceous material obtained by carbonizing a highly polymerized compound by heating can be used.
  • the form of the carbon (D) is not particularly limited. Between the short carbon fibers (A) described later may be bound by carbon having a mesh-like form, between the short carbon fibers (A) may be bound by a carbonized resin, and it is also possible to use a combination of these. In addition, in a case of the carbon (D) being a carbonized resin, it is possible to use a heat carbonizable resin (f) as the source material thereof.
  • This heat carbonizable resin (f) can be selected as appropriate from known resins that can bind the between short carbon fibers (A) in the carbonizing stage. From the viewpoint of facilitating remaining as a conductive material after carbonization, a phenolic resin, epoxy resin, furan resin, pitch or the like is preferred as the resin (f), and a phenolic resin having a high carbonization rate upon carbonizing by heating is particularly preferable.
  • a phenolic resin a resol-type phenolic resin obtained by the reaction between phenols and aldehydes under the presence of an alkali catalyst can be used.
  • a phenolic resin of Novolak type exhibiting solid-state heat fusability produced by reaction between phenols and aldehydes under the presence of an acid catalyst by a known method, can be dissolved and mixed into a liquid phenolic resin of resol type, in this case, one of self crosslinking type containing a curing agent, e.g., hexamethylene diamine, is preferable.
  • a phenolic resin solution produced by dissolving in alcohol or a solution of ketones, a phenolic resin dispersion produced by dispersing in a dispersant such as water, or the like can be used as the phenolic resin.
  • the three-dimensional mesh-like carbon fibers (B) are fibers bonding the short carbon fibers (A), and can form a three-dimensional mesh-like structure by existing in a state forming a bent shape or a curved shape at the bonding part.
  • the two-dimensional mesh-like carbon fibers (C) are fibers bonding the short carbon fibers (A), exist in a state forming a bent shape or curved shape at the bonding part, and can form a mesh-like structure formed within a two-dimensional plane.
  • the porous electrode substrate of the present invention can be produced by the following such processes, for example.
  • the first production process is a method of sequentially performing:
  • it may be impregnated with the heat carbonizable resin (f) prior to step (4), or in addition to the above, it may be impregnated with the heat carbonizable resin (f) prior to step (4) .
  • These fibers (b1), (b1'), (b2) and (b2') as well as the resin (f) can function as the carbon (D) in the porous electrode substrate, after passing through carbonization treatment.
  • a second production process is a method of further performing a step (5) of hot press molding the porous electrode substrate precursor sheet X-4 at a temperature less than 200°C after step (3), but before step (4) in the above first production process.
  • a third production process is a method of further performing a step (6) of oxidization treating the hot press molded porous electrode substrate precursor sheet X-4 at a temperature of at least 200°C and less than 300°C after step (5) but before step (4) in the above second production process.
  • step (1) it is preferable to disperse the short carbon fiber precursors (b1) and/or fibrillar carbon fiber precursors (b1') together with the short carbon fibers (A1) in step (1), and to disperse the short carbon fiber precursors (b2) and/or fibrillar carbon fiber precursors (b2') together with the short carbon fibers (A2) in step (3) .
  • a precursor sheet X-2' having a three-dimensional entangled structure in which the short carbon fibers (A1) as well as the short carbon fiber precursors (b1) and/or fibrillar carbon fiber precursors (b1') are dispersed in step (1), and it is possible to obtain a precursor sheet X-3' not having a three-dimensional entangled structure in which the short carbon fibers (A2) as well as the short carbon fiber precursors (b2) and/or fibrillar carbon fiber precursors (b2') are dispersed in step (3).
  • the precursor sheet X-2' has a three-dimensional entangled structure due to being entanglement treated in step (2).
  • short carbon fiber precursors (b1) and short carbon fiber precursors (b2) may be the same or may be different.
  • fibrillar carbon fiber precursors (b1') and the fibrillar carbon fiber precursors (b2') may be the same or may be different.
  • the carbon (D) being a carbonized resin
  • the porous electrode substrate by impregnating the heat carbonizable resin (f) into the porous electrode substrate precursor sheet X-4, subsequently curing by heating and pressurizing, and then carbonizing.
  • a method using a throttling device or a method overlapping a resin film on the precursor sheet is preferable.
  • the method using a throttling device is a method configured so as to impregnate the precursor sheet with a resin solution, and then have the beam limiting device uniformly coat the entire carbon sheet with the uptake liquid, and adjusting liquid amount by changing the roll gap of the throttling device.
  • the relative viscosity being low, it is possible to use a spray method or the like.
  • the method using a resin film first temporarily coats the carbonizable resin (f) onto mold release paper to make a film of the carbonizable resin (f). It is a method that subsequently performs a hot pressing process to laminate the film onto the precursor sheet and transcribes the carbonizable resin (f).
  • the short carbon fiber precursors (b) can be obtained by cutting long fibers of the carbon fiber precursor to an appropriate length.
  • the fiber length of the short carbon fiber precursors (b) is preferably on the order of 2 to 20 mm in terms of dispersibility.
  • the cross-sectional shape of the short carbon fiber precursors (b) is not particularly limited, a shape having high circularity is preferable in terms of the mechanical strength after carbonizing, and the production cost.
  • the diameter of the short carbon fiber precursors (b) is preferably no more than 5 ⁇ m in order to suppress fracture from shrinking during carbonization.
  • a polymer can be used as the material of the short carbon fiber precursors (b), and it is preferable to use a polymer having a residual mass of at least 20% by mass in the step of carbonization treatment.
  • Acrylic polymers, cellulose-based polymers, and phenolic polymers can be exemplified as such a polymer.
  • an acrylic polymer containing at least 50% by mass acrylonitrile units it is preferable to use an acrylic polymer containing at least 50% by mass acrylonitrile units.
  • the short carbon fiber precursors (b) may use one type independently, or may jointly use a plurality of types having different fiber diameters and polymers.
  • the mixing ratio with the short carbon fibers (A) and the presence of oxidation treatment (step (6)) under at least 200°C and no higher than 300°C, the proportion remaining as the three-dimensional mesh-like carbon fibers (B) or two-dimensional mesh-like carbon fibers (C) in the porous electrode substrate finally obtained will differ.
  • fibrillar carbon fiber precursors (b') for example, fibers produced by beating treating a carbon precursor fiber (b'-1) having a structure in which a plurality of fibrils having a diameter of tens of micrometers (e. g. , 0.1 to 3 ⁇ m) branches from a fibrous stem with a diameter on the order of 0.1 to 10 ⁇ m (hereinafter may be simply referred to as "fibers (b'-1)”), and short carbon fiber precursors (b'-2) made into fibrils by beating (hereinafter may be simply referred to as "fibers (b'-2)”) can be exemplified.
  • fibers (b'-1) having a structure in which a plurality of fibrils having a diameter of tens of micrometers (e. g. , 0.1 to 3 ⁇ m) branches from a fibrous stem with a diameter on the order of 0.1 to 10 ⁇ m
  • fibers (b'-2) short carbon fiber precursors (b'-2) made into fibrils
  • the short carbon fibers (A) and fibrillar carbon fiber precursors (b') are well intertwined inside the precursor sheet, whereby obtaining a precursor sheet excelling in handling property and mechanical strength is facilitated.
  • the freeness of the fibrillar carbon fiber precursors (b') is not particularly limited, generally, the mechanical strength of the precursor sheet improves when using fibrillar fibers having low freeness; however, there is a tendency for the gas permeability of the porous electrode substrate to decline.
  • one type of fiber (b'-1) or one type produced by beating treatment of the fiber (b'-2) may be used, and a plurality of types of these fibers having different freeness, fiber diameter, polymer type, etc. may be jointly used.
  • two or more types of fibers (b'-1) can be jointly used
  • two or more types of fibers produced by beating treating fibers (b'-2) can be jointly used, or alternatively, it is possible to jointly use at least one type of fiber (b'-1) and at least one type of fiber produced by beating treating the fiber (b'-2).
  • the polymer used in the fiber (b'-1) preferably has a residual mass in the carbonization treatment step of at least 20% by mass.
  • Acrylic polymers, cellulose polymers and phenolic polymers can be exemplified as such a polymer.
  • an acrylic polymer containing at least 50% by mass acrylonitrile units is preferable.
  • the average fiber length of the fibers (b'-1) is 1 to 20 mm.
  • Fibers produced by cutting an easy-to-split sea-island composite fiber in long fiber form to an appropriate length can be used as the fibers (b' -2). Such fibers can be beat with a refiner, pulper or the like to make into fibrils.
  • the fiber (b'-2) can be produced using at least two different kinds of polymers that are immiscible dissolved in a common solvent, and in this case, at least one type of polymer preferably has a residual mass in the carbonization treatment step of at least 20% by mass.
  • acrylic polymers As one having a residual mass of at least 20% by mass in the carbonization treatment step among the polymers used in the easy-to-split sea-island composite fibers, acrylic polymers, cellulose-based polymers and phenolic polymers can be exemplified. From the viewpoint of spinnability and residual mass in the carbonization treatment step, thereamong, it is preferable to use an acrylic polymer containing at least 50% by mass acrylonitrile units.
  • acrylic polymer that can be used in the fiber (b) and fiber (b'), it may be a homopolymer of acrylonitrile or a copolymer of acrylonitrile and another monomer.
  • the monomer copolymerized with acrylonitrile it is not particularly limited so long as being an unsaturated monomer constituting a general acrylic fiber; however, for example, acrylates typified by methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, and the like; methacrylates typified by methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-hexyl methacrylate, n
  • the weight-average molecular weight of the acrylic polymer is not particularly limited, and is preferably 50,000 to 1,000,000. There is a tendency for the yarn quality of the fiber to be good simultaneously with the spinnability improving, by the weight-average molecular weight of the acrylic polymer being at least 50,000. There is a tendency for the polymer concentration attributing to the optimum viscosity of the spinning dope to rise and the productivity to improve, by the weight-average molecular weight of the acrylic polymer being no more than 1,000,000.
  • the fiber (b'-2) in the case of using the aforementioned acrylic polymer as the polymer having a residual mass in the carbonization treatment step of at least 20% by mass, it is desirable to dissolve another polymer in a common solvent with this acrylonitrile-based polymer, and the spinning dope in which both polymers are dissolved to exist stably.
  • the fibers in a case of the extent of immiscibility of the two polymers in the spinning dope being large, the fibers may be of uneven quality, and fiber formation may not be possible due to thread breakage during spinning.
  • a polymer in the case of dissolving the other polymer in a common solvent with the acrylonitrile-based polymer, a polymer is desirable that has enough compatibility to be able to form a sea-island structure upon spinning, but is immiscible in the acrylonitrile-based polymer.
  • wet spinning if the other polymer dissolves in water in a solidification tank or in a washing tank, loss occurs leading to disadvantages in production; therefore, it is preferable for the other polymer to be insoluble in water.
  • polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polyvinylpyrrolidone, cellulose acetate, acrylic resins, methacrylic resins, phenolic resins, etc. can be exemplified; however, cellulose acetate, acrylic resin and methacrylic resins are preferable in the aspect of the balance of the aforementioned requirements.
  • the other polymer may be one type or may be two or more types.
  • the easy-to-split sea-island composite fiber used as the fiber (b'-2) can be produced by a common wet spinning method.
  • the spinning dope is prepared by dissolving the acrylonitrile-based polymer and the other polymer in a solvent.
  • the spinning dope may be made by mixing, in a static mixer or the like, the spinning dope obtained by dissolving the acrylonitrile-based polymer in solvent and the spinning dope obtained by dissolving the other polymer in solvent.
  • the solvent dimethylamide, dimethylformamide, dimethylsulfoxide or the like can be used.
  • the easy-to-split sea-island composite fiber can be obtained by supplying these spinning dopes to a spinning machine and spinning from nozzles, then conducting wet hot drawing, washing, drying and dry hot drawing.
  • the cross-sectional shape of the fiber (b'-2) is not particularly limited. From the viewpoint of dispersibility, the fineness of the fiber (b'-2) is preferably 1 to 10 dtex, in order to suppress breakage due to shrinking during carbonization.
  • the average fiber length of the fiber (b'-2) is preferably 1 to 20 mm from the viewpoint of dispersibility after beating.
  • the fiber (b'-2) is beaten by peeling off a phase-separated interface with a mechanical external force, and at least a portion thereof splits to form fibrils.
  • the beating method is not particularly limited; however, it is possible to form fibrils by a refiner or pulper, a beater, or spraying of a pressurized water stream (water-jet punching).
  • Upon beating the fibers (b'-2) by peeling off of the phase-separated interface with a mechanical external force the state of fibrillation changes depending on the beating method and beating duration.
  • the freeness evaluation ISO-5267-2 (Canadian Standard Freeness Method)
  • the freeness of the fibers (b'-2) is not particularly limited.
  • the precursor sheet X-1 can be obtained by dispersing the short carbon fibers (A1) and the short carbon fiber precursors (b1) and/or fibrillar carbon fiber precursors (b1'), and does not have a three-dimensional entangled structure.
  • the precursor sheet X-3 can be obtained by dispersing the short carbon fibers (A2) and the short carbon fiber precursors (b2) and/or fibrillar carbon fiber precursors (b2'), and does not have a three-dimensional entangled structure.
  • the short carbon fibers (A) are dispersed within a two-dimensional plane. In other words, the short carbon fibers (A1) in the precursor sheet X-1 are dispersed within a two-dimensional plane, and the short carbon fibers (A2) in the precursor sheet X-3 are dispersed within a two-dimensional plane.
  • the mass ratio of the short carbon fibers (A1) to the short carbon fiber precursors (b1) and fibrillar carbon fiber precursors (b1') in the precursor sheet X-1 is preferably 20:80 to 80:20 from the viewpoint of ensuring porous electrode substrate handling property after the carbonization treatment.
  • the mass ratio of the short carbon fibers (A2) to the short carbon fiber precursors (b2) and fibrillar carbon fiber precursors (b2') in the precursor sheet X-3 is preferably 20:80 to 80:20 from the viewpoint of ensuring porous electrode substrate handling properties after the carbonization treatment.
  • the precursor sheets X-1 and X-3 can be produced by a wet method or dry method.
  • the wet method is a method of sheet forming a precursor sheet by dispersing the short carbon fibers (A) with the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') in a liquid medium.
  • the dry method is a method of obtaining a precursor sheet by dispersing the short carbon fibers (A) with the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') in air and allowing to fall.
  • a medium in which the fiber short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') do not dissolve such as water and alcohol can be exemplified, for example; however, water is preferable from the viewpoint of productivity.
  • the precursor sheets X-1 and X-3 can be produced by either a continuous method or batch method; however, it is preferable to produce by a continuous method from the viewpoint of the productivity and mechanical strength of the precursor sheets X-1 and X-3.
  • the precursor sheet X-2 is obtained by subjecting the precursor sheet X-1 to entanglement treatment.
  • the entanglement treatment to entanglement the short carbon fibers (A) with the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') in the precursor sheet X-1 can be implemented by a known method so long as being a method in which a three-dimensional entangled structure is formed.
  • a mechanical entangling method such as a needle punching method, a high-pressure liquid spray method such as a water-jet punching method, a high-pressure gas spraying method such as steam-jet punching, or a method by a combination of these can be used.
  • a high-pressure liquid jet processing method is preferable from the aspects of being able to suppress breakage of the short carbon fibers (A) in the entangling step, and sufficient entangling being obtained.
  • the high-pressure liquid jet processing method is a processing method in which a precursor sheet is placed on a support member with a substantially smooth surface, and entangling the short carbon fibers (A) with the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') in the precursor sheet by causing a columnar liquid jet, fan-shaped liquid jet, slit liquid jet or the like sprayed at a pressure of 1 MPa to act thereon, for example.
  • any support member can be used so long as the design of the support member is not formed in the obtained structure having a three-dimensional entangled structure, and the sprayed liquid is quickly removed therefrom.
  • a 30 to 200 mesh wire net or plastic net, a roll, or the like can be exemplified.
  • the precursor sheet X-2 having a three-dimensional entangled structure by high-pressure liquid jet processing, after having continuously produced the precursor sheet X-1 on the support member with a substantially smooth surface.
  • any solvent that does not dissolve the fibers constituting the precursor sheet X-1 is acceptable, it is usually preferable to use water.
  • the orifice size of the respective spray nozzles in the high-pressure liquid jet nozzle is preferably in the range of 0.06 to 1.0 mm, and more preferably in the range of 0.1 to 0.3 mm in the case of being a columnar stream.
  • the distance between the nozzle jet orifice and the precursor sheet X-1 is preferable in the range of about 0.5 to 5 cm.
  • the pressure of the liquid is preferably at least 1 MPa and no more than 7 MPa, and more preferably at least 1 MPa and no more than 5 MPa.
  • the entanglement treatment may be performed in one line, or may be performed in a plurality of lines. In the case of performing in a plurality of lines, it is effective to increase the pressure of the high-pressure liquid jet processing of the second and later lines over the first line.
  • the entanglement treatment of the precursor sheet X-1 by a high-pressure liquid jet may be repeated a plurality of times.
  • another precursor sheet X-1 on the precursor sheet X-1 subjected to high-pressure spray processing is layer stacked, and then the high-pressure liquid jet processing may be performed.
  • the precursor sheet X-1 may be turned over while the three-dimensional entangled structure is being formed by the high-pressure liquid jet, and the high-pressure liquid jet processing may be further performed from the opposite side. In addition, these operations may be repeated.
  • the formation of striped track patterns resulting from the high-pressure liquid jet processing of the sheet in the sheet-forming direction can be suppressed by causing the high-pressure liquid jet nozzles provided with one line or a plurality of lines of nozzle orifices to oscillate in the width direction of the sheet.
  • the high-pressure liquid jet nozzles provided with one line or a plurality of lines of nozzle orifices to oscillate in the width direction of the sheet.
  • the porous electrode substrate precursor sheet X-4 is produced by layer stacking and integrating the precursor sheet X-3 not having a three-dimensional entangled structure on the precursor sheet X-2 having a three-dimensional entangled structure.
  • a method of layer stacking and integrating a method of separately producing the precursor sheet X-2 and the precursor sheet X-3, respectively, and then overlapping, a method of directly producing the precursor sheet X-3 on the precursor sheet X-2, and the like can be exemplified. Due to the bonding between the precursor sheet X-2 and the precursor sheet X-3 being easy, and further, the bonding force between sheets being strong, the method of directly producing the precursor sheet X-3 on the precursor sheet X-2 is preferable.
  • the porous electrode substrate precursor sheet X-4 in which the precursor sheet X-2 having a three-dimensional entangled structure and the precursor sheet X-3 not having a three-dimensional entangled structure are layer stacked and integrated can be obtained by directly feeding, onto the precursor sheet X-2 produced in advance, a slurry in which the short carbon fibers (A2) as well as the short carbon fiber precursors (b2) and/or fibrillar carbon fiber precursors (b2') are dispersed in a liquid medium to form a sheet.
  • the porous electrode substrate precursor sheet X-4 can also be obtained by layer stacking a plurality of the precursor sheets X-3 on the precursor sheet X-2.
  • the basis weight of the porous electrode substrate precursor sheet X-4 is preferably on the order of 10 to 200 g/m 2 , and the thickness is preferably on the order of 20 to 400 ⁇ m. It should be noted that the basis weight of the precursor sheet X-3 not having a three-dimensional entangled structure is preferably no more than 70% that of the porous electrode substrate precursor sheet X-4 in the aspect of raising the handling property of the porous electrode substrate precursor sheet X-4 and the porous electrode substrate, and is preferably at least 20% that of the porous electrode substrate precursor sheet X-4 in the aspect of reducing the damage to the polyelectrolyte membrane upon incorporating into the fuel cell of the porous electrode substrate. In other words, the basis weight of the precursor sheet X-2 having a three-dimensional entangled structure is preferably 30 to 80% that of the porous electrode substrate precursor sheet X-4.
  • the porous membrane base material precursor sheet X-4 can be carbonization treated as is, can be carbonization treated after hot press molding, and can be carbonization treated after oxidation treatment following the hot press molding.
  • the production cost can be curbed by carbonization treating as is.
  • the mechanical strength and conductivity of the obtained porous electrode substrate can be raised by causing the short carbon fibers (A) to fuse by the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b'1), and establishing a three-dimensional mesh-like carbon fibers (B) or two-dimensional mesh-like carbon fibers (C) by carbonizing the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b').
  • the carbonization treatment is preferably performed in inert gas in order to raise the conductivity of the porous electrode substrate.
  • the carbonization treatment is usually performed at a temperature of at least 1000°C.
  • the temperature range of the carbonization treatment is preferably 1000 to 3000°C, and more preferably 1000 to 2200°C.
  • the time of the carbonization treatment is on the order of 10 minutes to 1 hour, for example.
  • a pretreatment can be performed prior to the carbonization treatment by calcining in an inert atmosphere on the order of 300 to 800°C.
  • porous electrode substrate precursor sheet X-4 In a case of carbonization treating the porous electrode substrate precursor sheet X-4 produced continuously, it is preferable to perform carbonization treatment continuously over the entire length of the porous electrode substrate precursor sheet X-4, from the viewpoint of a production cost reduction. If the porous electrode substrate is long, since the handling property will improve, the productivity of the porous electrode substrate will rise, and the subsequent production of a membrane electrode assembly (MEA) can also be performed continuously, it is possible to reduce the production cost of the fuel cell. From the viewpoints of the productivity of the porous electrode substrate and fuel cell and a production cost reduction, it is preferable to continuously roll up the produced porous electrode substrate.
  • MEA membrane electrode assembly
  • the hot press molding can adopt any technique so long as being a technique that can uniformly hot press mold the porous electrode substrate precursor sheet X-4. For example, a method of hot pressing by placing flat rigid plates against both sides of the porous electrode substrate precursor sheet X-4, and a method using a continuous roll press machine or a continuous belt press machine can be exemplified.
  • a method using a continuous roll press machine or a continuous belt press machine is preferable. Continuously performing carbonization treatment is thereby facilitated.
  • a method applying pressure by with linear pressure to the belt by way of a roll press a method of pressing with specific pressure by way of a hydraulic head press, and the like can be exemplified. The latter is preferable in the aspect of a smoother porous electrode substrate being obtained.
  • the temperature during hot press molding is preferably less than 200°C, and more preferably 120 to 190°C.
  • the pressure during the hot press molding is not particularly limited, it is preferably on the order of 20 kPa to 10 MPa from the viewpoint of short carbon fiber (A) breakage prevention during hot press molding, and the viewpoint of porous electrode substrate densification prevention.
  • the content ratio of the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') in the porous electrode substrate precursor sheet X-4 being large, the surface of the precursor sheet Y can be easily smoothened even if the molding pressure is low.
  • the duration of the hot press molding can be set to 30 seconds to 10 minutes, for example.
  • hot press molding the porous electrode substrate precursor sheet X-4 by sandwiching between two rigid plates or with a continuous roll press machine or continuous belt press machine, it is preferable to spread mold release agent thereon beforehand, or to interpose mold release paper between the precursor sheet and the rigid plates, roll or belt, so that the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b'), etc. do not adhere to the rigid plates, roll or belt.
  • the short carbon fibers (A) by the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b') being satisfactorily carried out, and improving the carbonization rate of the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b')
  • the oxidation treatment is more preferably performed at 240 to 270°C.
  • Continuous oxidation treatment by way of a pressurized direct heating using a heated porous plate, or continuous oxidation treatment by way of intermittent pressurized direct heating using a heated roller or the like is preferable in the aspect of being low cost and being able to fuse the short carbon fibers (A) with the short carbon fiber precursors (b) and/or fibrillar carbon fiber precursors (b').
  • the duration of the oxidation treatment can be set to 1 minute to 2 hours, for example.
  • MEA Membrane Electrode Assembly
  • the porous electrode substrate of the present invention can be suitably used in a membrane electrode assembly.
  • Membrane electrode assemblies are composed of a polymer electrolyte membrane, catalyst layer and porous carbon electrode base material, providing a cathode-side catalyst layer composed of an oxidizing gas catalyst on one side of the polymer electrolyte membrane having proton conductivity, and providing an anode-side catalyst layer composed of a fuel gas catalyst on the other side, and in which a cathode-side porous electrode substrate and anode-side porous electrode substrate are provided on the outer sides of the respective catalyst layers.
  • a three-dimensional structure Y-2 side of the porous electrode substrate not having a three-dimensional entangled structure at a surface contacting the polymer electrolyte membrane.
  • the membrane-electrode assembly of the present invention can be suitably used in a polymer electrolyte fuel cell.
  • the polymer electrolyte fuel cell includes a cathode-side separator on which cathode-side gas channels are formed, and an anode-side separator on which anode-side gas channels are formed, so as to sandwich the membrane electrode assembly.
  • an oxidizing gas inlet and oxidizing gas outlet, and fuel gas inlet and fuel gas outlet are provided to the respective separators.
  • a porous electrode substrate excelling in handling property, having improved sheet undulation, as well as retaining sufficient gas permeability and electrical conductivity, and further, does not inflict damage on the polymer electrolyte membrane upon incorporating in the fuel cell.
  • the method of producing a porous electrode substrate of the present invention it is possible to produce the porous electrode substrate at low cost.
  • Parts indicates “parts by mass”.
  • the time required for 200 mL of air to permeate was measured using a Gurley Densometer, and the gas permeability (ml/hr/cm 2 /Pa) of the porous electrode substrate was calculated.
  • the thickness of the porous electrode substrate was measured using a dial thickness gauge (trade name: 7321, manufacturing by Mitutoyo Corp.).
  • the size of the gauge head was 10 mm in diameter, and the measurement pressure was set to 1.5 kPa.
  • the resistance value was measured when sandwiching the porous electrode substrate between gold plated copper plates, pressurizing from above and below the copper plates at 1 MPa, and flowing current at an current density of 10 mA/cm 2 , and the electrical resistance (through-plane resistance) in the thickness direction of the porous electrode substrate was obtained from the following equation.
  • Through - plane resistance m ⁇ ⁇ cm 2 measured resistance value m ⁇ ⁇ sample surface area cm 2
  • the total content of the three-dimensional mesh-like carbon fiber (B) and two-dimensional mesh-like carbon fiber (C) was calculated according to the following formula from the basis weight of the obtained porous electrode substrate and the basis weight of the short carbon fibers (A) used.
  • Total content (mass %) of three-dimensional mesh-like carbon fiber (B) and two-dimensional mesh-like carbon fiber (C) [porous electrode substrate basis weight (g/m 2 ) - short carbon fiber (A) basis weight (g/m 2 ) / porous electrode substrate basis weight (g/m 2 ) x 100
  • the undulation of the porous electrode substrate was calculated from the difference between the maximum value and minimum value of the height when laying a porous electrode substrate with a height of 250 mm and width of 250 mm on a flat plate.
  • a perfluorosulfonic acid-based polymer electrolyte membrane (membrane thickness: 30 ⁇ m) on which catalyst layers (catalyst layer surface area: 25 cm 2 , Pt deposit per unit area: 0.3 mg/cm 2 ) composed of catalyst loaded carbon (catalyst: Pt, catalyst loading: 50% by mass) were formed on both sides was sandwiched between two porous electrode substrates so that the three-dimensional structure side not having a three-dimensional entangled structure contacted with the polymer electrolyte membrane, and these were bonded to obtain an MEA.
  • This MEA was interposed by the two carbon separators having bellows-like gas channels to prepare a polymer electrolyte fuel cell (unit cell). Then, by measuring the open circuit voltage (OCV) when supplying hydrogen gas and air through bubblers at 80°C to the unit cell with the temperature set to 80°C, the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was confirmed.
  • OCV open circuit voltage
  • PAN-based carbon fiber having an average fiber diameter of 7 ⁇ m and average fiber length of 3 mm was prepared as the short carbon fiber (A).
  • the short carbon fiber precursor (b) acrylic short fiber having an average fiber diameter of 4 ⁇ m and average fiber length of 3 mm was prepared (trade name: D122, manufactured by Mitsubishi Rayon Co., Ltd.), and as the fibrillar carbon fiber precursor (b'), easy-to-split acrylic sea-island composite fiber (b'-2) composed of diacetate (cellulose acetate) and acrylic polymer to be fibrillated by beating was prepared (trade name: VONNEL M.V.P-C651, average fiber length: 3 mm, manufactured by Mitsubishi Rayon Co., Ltd.)
  • the precursor sheet X-1, precursor sheet X-2 having a three-dimensional entangled structure, and the porous electrode substrate precursor sheet X-4 produced by layer stacking and integrating the precursor sheet X-3 not having a three-dimensional entangled structure onto the precursor sheet X-2 were continuously produced to obtain a carbon electrode base material.
  • the short carbon fibers (A) were dispersed in water so that the fiber concentration was 1% by mass (10 g/L), and were defibration treated through a disk refiner (manufactured by Kumagai Riki Kogyo Co., Ltd.) to provide defibrated slurry fibers (SA).
  • the short carbon fiber precursors (b) were dispersed in water so that the fiber concentration was 1% by mass (10 g/L), and were defibration treated through a disk refiner (manufactured by Kumagai Riki Kogyo Co., Ltd.) to provide defibrated slurry fibers (Sb).
  • the easy-to-split acrylic sea-island composite short fibers were dispersed in water so that the fiber concentration was 1% by mass (10 g/L), and were subjected to beating and defibration treatment through a disk refiner (manufactured by Kumagai Riki Kogyo Co., Ltd.) to provide defibrated slurry fibers (Sb').
  • the defibrated slurry fibers (SA), defibrated slurry fibers (Sb), defibrated slurry fibers (Sb') and dilution water were measured so that the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) to fibrillar carbon fiber precursors (b') was 50:30:20 and the concentration of fibers in the slurry was 1.44 g/L, and were charged into a slurry feed tank. Furthermore, polyacrylamide was added to prepare a sheet-forming slurry with a viscosity of 22 centipoise (22 mPa ⁇ s).
  • Treatment equipment including a sheet-shaped material conveying device made from a net driving unit, and a continuously rotatable net in which a plain-woven mesh made of a 60 cm wide by 585 cm long plastic net was connected in a belt shape; a sheet-forming slurry feed apparatus having a slurry feed portion width of 48 cm and a feed slurry amount of 30 L/min; and a reduced-pressure dewatering apparatus arranged under the net.
  • the aforementioned sheet-forming slurry was fed above the plain-woven mesh by a metering pump.
  • the sheet-forming slurry was widened to a predetermined size through a flow box for rectifying to a uniform flow, and then fed. Subsequently, it was left to stand, passed through a natural dewatering portion, and then dewatered by the reduced-pressure dewatering apparatus, thereby obtaining the precursor sheet X-1.
  • the target basis weight of the precursor sheet X-1 was set to 35 g/m 2 .
  • the pressurized water stream jet treatment apparatus including three water jet nozzles shown in the below Table 1 was arranged.
  • Table 1 Orifice size Pitch between orifices (width direction) Arrangement Nozzle effective width Nozzle1 ⁇ 0.15 mm ⁇ 501 orifices 1 mm 1 line arrangement 500mm (1001 orifices / 1 m width) Nozzle2 ⁇ 0.15 mm ⁇ 501 orifices 1 mm 1 line arrangement 500mm (1001 orifices / 1 m width) Nozzle3 ⁇ 0.15 mm ⁇ 1002 orifices 1.5mm 3 line arrangement Pitch between lines 5 mm 500mm
  • the precursor sheet X-1 was loaded onto a net of the pressurized water stream jet treatment apparatus. Then, setting the pressurized water stream jet pressure to 1 MPa (nozzle 1), 2 MPa (nozzle 2) and 1 MPa (nozzle 3), the precursor sheet X-1 was subjected to entanglement treatment by being passed in the order of the nozzle 1, nozzle 2 and nozzle 3, thereby obtaining the precursor sheet X-2 having a three-dimensional entangled structure. It should be noted that the target basis weight of the precursor sheet X-2 having a three-dimensional entangled structure is 35 g/m 2 , which is the same as the target basis weight of the precursor sheet x-1.
  • treatment equipment treatment equipment including sheet-like material conveying apparatus, sheet-forming slurry feed apparatus, and reduced-pressure dewatering apparatus arranged under net
  • treatment equipment similar to the treatment equipment used in the production of the precursor sheet X-1 were arranged.
  • the aforementioned sheet-forming slurry was fed by a metering pump from above the precursor sheet X-2 having a three-dimensional entangled structure loaded onto the plain-woven mesh.
  • the sheet-forming slurry was widened to a predetermined size through a flow box for rectifying to a uniform flow, and then fed.
  • the precursor sheet X-3 not having a three-dimensional entangled structure was layer stacked, thereby obtaining the porous electrode substrate precursor sheet X-4 in which the precursor sheet X-2 having a three-dimensional entangled structure and the precursor sheet X-3 not having a three-dimensional entangled structure are layer stacked and integrated. It should be noted that, since the target basis weight of the precursor sheet X-3 was set to 35 g/m 2 , the target basis weight of the porous electrode substrate precursor sheet X-4 is 70 g/m 2 .
  • the porous electrode substrate precursor sheet X-4 was dried for 3 minutes at 150°C by a pin tenter tester (trade name: PT-2A-400, manufactured by Tsuji Dyeing Machine Manufacturing Co., Ltd.).
  • the basis weight of the porous electrode substrate precursor sheet X-4 was 70.2 g/m 2 .
  • the dispersed state of the short carbon fibers (A), short carbon fiber precursors (b) and fibrillar carbon fiber precursors (b') in this porous electrode substrate precursor sheet X-4 was favorable, and further, e of fibers in the precursor sheet X-2 was favorable, and the handling property was also favorable.
  • both surfaces of the porous electrode substrate precursor sheet X-4 were interposed between paper coated with a silicone-based mold release agent, and then, was hot press molded for 3 minutes under conditions of 180°C and 3 MPa in a batch press machine.
  • porous electrode substrate precursor sheet X-4 was carbonization treated in a batch carbonization furnace in a nitrogen gas atmosphere under the condition of 2000°C to obtain the porous electrode substrate.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the total content of the three-dimensional mesh-like carbon fibers (B) and two-dimensional mesh-like carbon fibers (C) was 24% by mass.
  • scanning electron micrographs of cross sections of the obtained porous electrode substrate are shown in FIG. 1 (cross section of three-dimensional structure having three-dimensional entangled structure) and FIG. 2 (cross section of three-dimensional structure not having three-dimensional entangled structure).
  • scanning electron micrographs of front and back surfaces of the obtained porous electrode substrate are shown in FIGS. 3 and 4 .
  • FIG. 1 it could be confirmed that the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and in FIG. 2 , it could be confirmed that the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C). Since the A surface does not have an entangled structure in FIG. 3 , it could be confirmed that fluffing of the short carbon fibers (A), carbonized acrylic fibers, is suppressed. On the other hand, since the B surface has an entangled structure, fibers projecting from the surface could be observed in FIG. 4 (locations with round border).
  • the OCV of a unit cell using this porous electrode substrate was high at 0.902 V, and the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was low.
  • the above evaluation results are shown in Table 2 along with the basis weight of the porous electrode substrate.
  • Porous electrode substrates were obtained similarly to Example 1, except for setting the target basis weights of the precursor sheet X-2 having a three-dimensional entangled structure and the precursor sheet X-3 not having a three-dimensional entangled structure to 25 g/m 2 and 45 g/m 2 (Example 2), or 55 g/m 2 and 15 g/m 2 (Example 3), respectively.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • Porous electrode substrates were obtained similarly to Example 1, except for setting the target basis weights of the precursor sheet X-2 having a three-dimensional entangled structure, the precursor sheet X-3 not having a three-dimensional entangled structure, and the porous electrode substrate precursor sheet X-4 layer stacking and integrating these to 30 g/m 2 , 30 g/m 2 and 60 g/m 2 (Example 4), or 20 g/m 2 , 20 g/m 2 and 40 g/m 2 (Example 5), respectively.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) to fibrillar carbon fiber precursors (b') in the sheet-forming slurry to 50:40:10.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) to fibrillar carbon fiber precursors (b') in the sheet-forming slurry to 40:40:20, and setting the target basis weights of the precursor sheet X-2 having a three-dimensional entangled structure, the precursor sheet X-3 not having a three-dimensional entangled structure, and the porous electrode substrate precursor sheet X-4 layer stacking and integrating these to 40 g/m 2 , 40 g/m 2 and 80 g/m 2 , respectively.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) to fibrillar carbon fiber precursors (b') in the sheet-forming slurry to 30:50:20, and setting the target basis weights of the precursor sheet X-2 having a three-dimensional entangled structure, the precursor sheet X-3 not having a three-dimensional entangled structure, and the porous electrode substrate precursor sheet X-4 layer stacking and integrating these to 45 g/m 2 , 45 g/m 2 and 90 g/m 2 , respectively.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) to fibrillar carbon fiber precursors (b') in the sheet-forming slurry to 70:10:20, and setting the target basis weights of the precursor sheet X-2 having a three-dimensional entangled structure, the precursor sheet X-3 not having a three-dimensional entangled structure, and the porous electrode substrate precursor sheet X-4 layer stacking and integrating these to 30 g/m 2 , 30 g/m 2 and 60 g/m 2 , respectively.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the pressurized water stream jet pressure to 2 MPa (nozzle 1), 3 MPa (nozzle 2) and 2 MPa (nozzle 3).
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the pressurized water stream jet pressure to 3.5 MPa (nozzle 1), 4.5 MPa (nozzle 2) and 3.5 MPa (nozzle 3).
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for using a polyacrylonitrile-based pulp (b'-1) in which several fibrils having a diameter of no more than 3 ⁇ m branch from a fibrous stem as the fibrillar carbon fiber precursor (b'). It should be noted that the polyacrylonitrile-based pulp (b'-1)was produced by jet solidification.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C) .
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 12, except for repeatedly conducting the three-dimensional entanglement treatment by way of a pressurized water stream jet twice from the same surface.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C) .
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 12, except for implementing the pressurized water stream jet again from the back surface after conducting the three-dimensional entanglement treatment by way of the pressurized water stream jet from the top surface.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for not using the fibrillar carbon fiber precursors (b'), and setting the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) in the sheet-forming slurry to 50:50.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for not using the short carbon fiber precursors (b), and setting the mass ratio of the short carbon fibers (A) to fibrillar carbon fiber precursors (b') in the sheet-forming slurry to 50:50.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C) .
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 16, except for using a polyacrylonitrile-based pulp (b'-1) in which several fibrils having a diameter of no more than 3 ⁇ m branch from a fibrous stem as the fibrillar carbon fiber precursor (b'). It should be noted that the polyacrylonitrile-based pulp (b'-1) was produced by jet solidification.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for interposing both surfaces of a hot press molded porous electrode substrate precursor sheet X-4 with stainless-steel punching plates coated with a silicone-based mold release agent, and then oxidation treating in a batch press machine in the atmosphere under conditions of 280°C and 0.5 MPa, prior to the carbonization treatment.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for not having carried out hot press molding.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) to fibrillar carbon fiber precursors (b') in the sheet-forming slurry to 20:30:50, and setting the target basis weights of the precursor sheet X-2 having a three-dimensional entangled structure, the precursor sheet X-3 not having a three-dimensional entangled structure, and the porous electrode substrate precursor sheet X-4 layer stacking and integrating these to 45 g/m 2 , 45 g/m 2 and 90 g/m 2 , respectively.
  • the obtained porous electrode substrate had an external appearance in which wrinkles were formed due to in-plane shrinking during the carbonization treatment; however, the undulation was small at 3 mm, the surface smoothness was also favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C) .
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for setting the mass ratio of the short carbon fibers (A) to short carbon fiber precursors (b) to fibrillar carbon fiber precursors (b') in the sheet-forming slurry to 80: 10: 10, and setting the target basis weights of the precursor sheet X-2 having a three-dimensional entangled structure, the precursor sheet X-3 not having a three-dimensional entangled structure, and the porous electrode substrate precursor sheet X-4 layer stacking and integrating these to 30 g/m 2 , 30 g/m 2 and 60 g/m 2 , respectively.
  • the obtained porous electrode substrate had an external appearance in which wrinkles were formed due to in-plane shrinking during the carbonization treatment; however, the undulation was small at 3 mm, the surface smoothness was also favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C) .
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for producing the layer stacked and integrated precursor sheet by separately producing the precursor sheet X-2 having a three-dimensional entangled structure and the precursor sheet X-3 not having a three-dimensional entangled structure, allowing to dry, and then overlapping the two and hot press molding in a batch press machine for 3 minutes under conditions of 180°C and 3 MPa. It should be noted that the precursor sheet X-3 was produced similarly to the production method of X-1 in Example 1.
  • the obtained porous electrode substrate had an external appearance in which wrinkles were formed due to in-plane shrinking during the carbonization treatment, the undulation was small at no more than 2 mm, the surface smoothness was also favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure in which the short carbon fibers (A) are bonded by the three-dimensional mesh-like carbon fibers (B), and a three-dimensional structure in which the short carbon fibers (A) are bonded by the two-dimensional mesh-like carbon fibers (C) .
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • a defibrated slurry was used that had been prepared similarly to the defibrated slurry Sb using polyvinyl alcohol (PVA) short fibers having an average fiber length of 3 mm (trade name: VBP105-1, manufactured by Kuraray Co., Ltd.). Then, the mass ratio of the short carbon fibers (A) to the polyvinyl alcohol (PVA) short fibers in the sheet-forming slurry was set to 80: 20. Otherwise, the porous electrode substrate precursor sheet X-4 was obtained similarly to Example 5.
  • PVA polyvinyl alcohol
  • the porous electrode substrate precursor sheet X-4 impregnated by phenolic resin was obtained by impregnating the porous electrode substrate precursor sheet X-4 with a methanol solution of the phenolic resin (trade name: Phenolite J-325, manufactured by Dainippon Ink and Chemicals, Inc.), and allowing the methanol to sufficiently dry at room temperature, so that the mass ratio of the porous electrode substrate precursor sheet X-4 to nonvolatile components of the phenolic resin was 50:50.
  • press-heat molding and carbonization treatment were performed at the same conditions of Example 1 to obtain a porous electrode substrate.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • the porous electrode substrate had a structure integrating and layer stacking a three-dimensional structure having an entangled structure in which the short carbon fibers (A) are bonded by the carbon (D), and a three-dimensional structure not having an entangled structure in which the short carbon fibers (A) are bonded by the carbon (D).
  • the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was also small. The above evaluation results are shown in Table 2.
  • An MEA was obtained similarly to the technique described in the aforementioned OCV measurement method (evaluation method for damage to the polymer electrolyte membrane upon incorporating in the fuel cell), using two of the porous electrode substrates obtained in Example 1 as the porous carbon electrode base materials for the cathode and anode.
  • the obtained MEA was interposed by two carbon separators having bellows-like gas channels to form a polymer electrolyte fuel cell (unit cell).
  • Evaluation of the fuel cell characteristics was performed by measuring the current density-voltage characteristics of this unit cell. Hydrogen gas was used as the fuel gas and air was used as the oxidizing gas. The temperature of the unit cell was set to 80°C, the fuel gas utilization rate was set to 60%, and the oxidizing gas utilization rate was set to 40%. In addition, humidification of the fuel gas and oxidizing gas was performed by passing the fuel gas and the oxidizing gas through bubblers at 80°C, respectively. As a result, the cell voltage of the fuel cell when the current density was 0.8 A/cm 2 was 0.644 V, and the internal resistance of the cell was 2.9 m ⁇ , which indicated favorable characteristics.
  • a porous electrode substrate was obtained similarly to Example 1, except for not having conducted the three-dimensional entanglement treatment by way of a pressurized water stream jet.
  • the precursor sheet X-3 was formed on the precursor sheet X-1, since neither has a three-dimensional entangled structure, the target basis weights of both were totaled and indicated in the field of "basis weight of X-3" in Table 2.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable; however, the handling property of the porous electrode substrate precursor sheet X-4 greatly declined.
  • three-dimensional mesh-like carbon fibers (B) were not observed in the porous electrode substrate, which had a structure in which the short carbon fibers (A) were bonded by the two-dimensional mesh-like carbon fibers (C) .
  • the damage to the polymer electrolyte membrane upon inserting in the fuel cell was small. The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for obtaining the porous electrode substrate precursor sheet X-4 by layer stacking and integrating the precursor sheet X-2 having a three-dimensional entangled structure and the precursor sheet X-3 not having a three-dimensional entangled structure, and then further conducting entanglement treatment by way of a pressurized water stream jet similarly to Example 1.
  • the target basis weight of both were totaled and indicated in the field of "basis weight of X-2" in Table 2.
  • the obtained porous electrode substrate had almost no in-plane shrinking during the carbonization treatment, the undulation of the sheet was small at no more than 2 mm and thus the surface smoothness was favorable, and the gas permeability, thickness and through-plane electric resistance were each favorable.
  • two-dimensional mesh-like carbon fibers (C) were not observed in the porous electrode substrate, which had a structure in which the short carbon fibers (A) were bonded by the three-dimensional mesh-like carbon fibers (B).
  • the OCV of the unit cell using this porous electrode substrate was low at 0.883 V, and thus the damage to the polymer electrolyte membrane upon incorporating in the fuel cell was large.
  • Table 2 The above evaluation results are shown in Table 2.
  • a porous electrode substrate was obtained similarly to Example 1, except for not using the short carbon fibers (A), and setting the mass ratio of the short carbon fiber precursors (b) to the fibrillar carbon fiber precursors (b') in the sheet-forming slurry to a mass ratio of 60:40.
  • the obtained porous electrode substrate had a large amount of in-plane shrinkage during the carbonization treatment, and the sheet form could not be retained.
  • a defibrated slurry was used that had been prepared similarly to the defibrated slurry Sb using polyvinyl alcohol (PVA) short fibers having an average fiber length of 3 mm (trade name: VBP105-1, manufactured by Kuraray Co., Ltd.). Then, the mass ratio of the short carbon fibers (A) to the polyvinyl alcohol (PVA) short fibers in the sheet-forming slurry was set to 80:20. Otherwise, the porous electrode substrate was obtained similarly to Example 1.
  • PVA polyvinyl alcohol

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